I have thought about this.
What I want is a game that doesn’t take place where Geralt has already been, but in all the worlds in between. We find her at the beginning of the game in a throne room or a cozy cottage (depending on how you ended the Wild Hunt) and learn she’s spent the last fifteen years since the Wild Hunt were defeated hopping between worlds and realizing there’s something afoot with the larger cosmology that holds everything together.
It doesn’t have to be that everything is going wrong, but that there’s a pattern of some kind that sits uneasy with her. So she has to travel between, say, four or five main realms and uncover the mystery of this pattern.
Also one of the realms she visits is her home world, back at the very beginning of the Witchers, when humans first came along and monsters still terrorized everyone.
I haven’t thought too much about the plot details but I’d love a story that takes her across all those places that gives players the opportunity to really live as Ciri, to get in her head, to learn about who she’s become as an adult, etc.
edit: The other reason I would have it take place in other worlds is to credibly increase the difficulty. By the end of the Wild Hunt you’re zipping through heavily armored knights like they’re level 1 Drowners. I’d love her to face some truly other-worldly, terrifying monsters.